This invention relates to a method of fabricating on one surface of a transparent substrate a thin film structure comprising a plurality of thin film layers in predetermined patterns, which method comprises the steps of providing a light shielding pattern adjacent the opposing surface of the substrate and photolithographically patterning a first thin film layer deposited over the one surface according to the light shielding pattern using radiation directed onto the light shielding pattern.
The invention relates also to thin film structures fabricated thereby and display devices incorporating such structures.
Thin film structures fabricated on transparent substrates are used in a variety of products such as display devices, solar cells, light sensing devices, memory devices, and printing devices. The invention is concerned with fabricating thin film structures suitable for such products and particularly, although not exclusively, with fabricating arrays of thin film switching elements for use in active matrix display devices, together with associated address conductors and picture element electrodes. Typically, the thin film switching elements comprise two or three terminal devices such as MIM devices and TFTs respectively which are connected between address conductors and picture element electrodes on a common transparent substrate.
A method of the kind described in the opening paragraph for fabricating thin film MIM devices on a transparent substrate for use in an active matrix liquid crystal display device is disclosed in U.S. Pat. No. 4,683,183. In this method, metal strips constituting a set of address conductors are defined on the substrate using a mask. Thereafter successive layers of insulating and conducting materials are deposited over the substrate surface and these layers are patterned by means of a photolithographic process which involves depositing a photoresist layer over the layers to be defined, applying a photomask to the rear surface of the substrate and illuminating the substrate from behind whereby the pattern of the superimposed insulating and conducting layers obtained is determined in part using a self alignment technique in which the metal strips serve as a mask to define a MIM structure and also in part by the photomask to define the length of the MIM structure and the area of the picture element electrode. Thus, the fabrication of the MIM device and its associated picture element electrodes entails using a mask to define the metal strips, those strips then themselves being used as a mask, and also a photomask.
In the English language abstract of Japanese Patent Application No. 62-132367 there is disclosed a method of fabricating TFTs on a transparent substrate for use in a liquid crystal display device which involves disposing a light shielding film layer on the rear surface of the substrate and photolithographically patterning a layer deposited on the upper surface by illuminating the substrate from behind with the light shielding film serving as a mask defining regions of the deposited layer which constitute source and drain terminals of the TFT. Further layers are then deposited which would require additional patterning processes using further masks to complete the TFT.
In requiring a plurality of patterning processes involving separate masks, these known methods suffer from disadvantages similar to those experienced with more common methods of fabricating of TFTs or MIMs for display devices or the like entailing a series of conventional photolithographic processes needing a set of several photo masks to define patterns in layers of different materials. To increase the display area of a display device, for example, it is necessary to produce correspondingly sized arrays of TFTs or MIMs. However, as the size of the array is enlarged it becomes increasingly more difficult and more expensive to provide substrates that remain sufficiently rigid through the thermal cycles in the fabrication processes to ensure accurate alignment between successive photomasks and the substrate. A similar problem exists when fabricating smaller sized but higher density arrays. Of course, such known methods are not well suited to fabricating thin film structures on flexible substrates.